Amplifying apparatus



Dec. 4, 1934.

v A. D. SILVA AMPLIFYING APPARATUS Filed July 26, 1924 INVENTOR. y M 5 div B Mk4. i 7 ATTORNEY.

Patented Dec. 4, 1934 AMPLIFYING APPARATUS Albert D. Silva, Philadelphia, Pa., assignor to Atwater Kent Manufacturing Company, Philadelphia, Pa., a corporation of- Pennsylvania Application July 26, 1924, Serial No. 728,280

1 Claim. (01. 179-171) My invention relates to thermionic amplifiers and their associated circuits ,utilizable for amplifying fluctuating currents or oscillations, and more particularly currents of the character pres- 5 ent in radio receiving apparatus.

I In accordance with my invention, the circuits of thermionic amplifying apparatus, and more particularly of tuned radio frequency amplifying apparatus involving a thermionic device or a plurality in cascade, have imparted thereto an electrical characteristic which suppresses inherent regenerative action between the input and output circuits, suppresses the production of oscillations,

and in general reduces or eliminates the effect of the inherent coupling caused by the capacity between the control electrode or grid and anode of a thermionic device.

In accordance with my invention, there is introduced into one of the circuits, particularly the "26 input circuit of a thermionic device, but external to the tuned circuitper se comprising substan-.

tially solelythe tuning inductanceand capacity, a resistance, or impedance of such magnitude as to reduce regenerative coupling between the out-, put and input circuitssand prevent production of oscillations; further in accordance withmy invention, the magnitude of the resistance or impedance is fixed oryconstant, and such as to serve for thermionic vacuum tubes varying among themselves as to their electrical constants or characteristics, for varying conditions of use of a vacuum tube, for? a relatively wide range of wave lengths, and such as to increase the damping of the aforesaid'tuned, circuit per se to a de- 5 gree not materially or seriously interfering with procurement of resonance or sharp tuning.

My invention resides in the hereinafter described features of structure and proportionment. For an understanding of my invention and for an illustration of one of the various forms itmay take, reference is to be had to the accompanying drawing, which is a diagrammatic view of a radio receiving system. I

The antenna D represents generically an an- 4 tenna, a loop antenna or any other structure for absorbing electro-radiant energy from the natural media, and represents also a circuit or path in or operatively related to an artificial conductor system extending between transmitting and re- 5 ceiving stations for transmitting'signalsjspeech or controls by carrier waves. r

Between the antenna D and the earth or counter-capacity E is connected the primary of a a radio frequency transformer T, whose secondary is shunted by the variable tuning condenser C,

shunted by the grid leak resistance 1'.

which with said secondary comprises the tuned circuit per se whose electrical constants are such as to make it resonant with or sharply tuned to the received high frequency energy. The thermionic device V, as a vacuum tube of the audion type, comprises as usual the anode a, grid or control electrode 9 and filament or cathode and has its input or grid circuit connected to the aforesaid tuned circuit. The anode and cathode at least are disposed in an envelope or tube evacuated to any suitable degree, and generally and preferably to that high degree effecting pure electron discharge. It will be understood, however, that my invention is applicable to any suitable degree of vacuum, .and is applicable also to those tubes containing gas. I

The tube V operates as an amplifier, in this ex ample as a radio frequency amplifier, in whose anode circuit is connected the primary of a radio frequency transformer T1 whose secondary constitutes an inductance shunted by the tuning condenser C1, which with its associated inductance constitutes a secondresonant or tuned circuit, connected to or in the input circuit of a second amplifier V1, in whose anode circuit is the primary of a third radio frequencytransformer T2 whose secondary is a tuning inductance shunted by the tuning capacityfCZ and forming therewith a tuned or resonant circuit connected-to or in the grid circuit of the detector tube V2, which may have characteristics similar to either of the tubes V, V1, or which may have different characteristics, as, ior example, lower vacuum, suiting it to demodulating or detector purposes. In circuit with the grid is the grid condenser K In the anodecircuit of the tube V2, which is not tuned, or is at most broadly tuned, is included the instrument I, which may be a telephone, loud. speaker or, generically, any translating device, as, for example, the" primary of an audio frequency transformerfor coupling the anode circuitof the tube V2 with the grid circuit of an audio frequency amplifier, which may be the first of any suitable number of stages of audio frequency amplification;

Thecathodes f are supplied, by current from the battery A, and the anode circuits from the battery or source of current 13.

Between the anode a andgrid g of a thermionic device of the audion type exists a capacity which couples or assist-sin coupling the output and inputcircuits to each other. This coupling capacity, indicated in dotted lines atC3, lends itself in effecting what may be a regenerative coupling loo cathode capacity, is greater.

? particularly their anode-to-cathode impedances;

? be sufficiently high to suit a maximum value between output and input circuits which in many cases causes the production locally of parasitic or undesired oscillations. The prevention or suppression of the regenerative coupling, and the prevention of the production of such oscillations are objects of my invention, which, in another aspect, relates to the reduction or annulment of the eifect of the intra-tube coupling capacity.

In accordance with my invention, I introduce into the input, control-electrode or grid circuit, externally, however, to the tuned or resonant circuit per se comprising the tuning capacity condenser C, C1 or C2, and either in the filament or grid lead, but preferably, as indicated, in the grid lead, a resistance or impedance R, which may be a non-inductive resistance, or an impedance comprising resistance and some inductive reactance, though a strictly non-inductive or substantially non-inductive resistance is preferred.

By utilization of a resistance R of suitable magnitude, the regenerative action between output and input circuits of a tube is reduced or annulled, the production of undesired or parasitic oscillations is prevented, and in general, the coupling eiiect of the grid-to-anode capacity C3 is materially reduced or annulled as regards its action in assisting regenerative coupling between the output and input circuits of the tube.

The utilization of the resistance R prevents impression upon the associated grid c of a potential of such magnitude and phase relation as to encourage or induce regenerative action or as to cause the production of undesired or parasitic oscillations.

The magnitude of the impedance or resistance R suitable for the purposes described should, from the aspect of its damping efiect upon the tuned or resonant circuit per se, be as small as possible to preserve to as great an extent as possible the sharp tuning qualities of that circuit, and, on the other hand, should be of a magnitude sufficiently great to reduce or eliminate the effect of regenerative action and production of oscillations.

The magnitude of the resistance or impedance R will be greater as the amplification factor of the associated tube or tubes is greater and as the grid-to-anode capacity, C3, is greater, and is greater as the reciprocal of the anode circuit resistance or impedance within the tube is greater, and particularly as the. impedance or resistance between the anode and cathode with respect to fluctuating or alternating current and as influenced to some extent by the anode- Tubes, even when of the same type or make, vary among themselves as regards the magnitudes of their amplification factors, grid-toanode capacities, and plate circuit impedances,

and furthermore, the relations of these factors vary with varying conditions under which the tubes are employed. In consequence, it is desirable that the magnitude of the resistance R of the product of the amplification factor, gridto-anode capacity and reciprocal of the anode circuit or anode-to-cathode impedance.

I have found that a suitable magnitude of the resistance or impedance R in ohms is determinable from the expression:-R=kmCPl0" in which, m is the amplification factor, C the gridto-plate capacity in micro-microfarads, k is a factor of the order of l for two or more amplifier tubes in cascade, and P the impedance of the plate circuit external to the tube.

Taking, for example, a UV 201A tube now upon the market, for which m is 8, C is 6, and with P 3000 ohms, R for each of the stages should be given a magnitude of the order of 55D to 650 ohms. For these conditions stated, a resistance R of this magnitude will prevent production of the undesirable oscillations and reduce or suppress the effect of inherent regenerative action, and yet will not materially aifect the sharpness of tuning of the tuned or resonant circuit per se comprising any of the condensers C, Cl and C2 With its associated inductance, which is, in the example illustrated, a secondary of a radio frequency transformer. However, UV 201-A tubes vary among themselves as to the aforesaid factors, and to take care of variations consisting in increases in the amplification factor and gridto-anode capacity and decrease in the impedance P, the resistance R is in practice preferably given a magnitude greater than its aforesaid value, but not sufficiently greater seriously to affect the sharp tuning or resonance qualities of the tuned circuit per se.

In case of employment of but a single radio frequency amplifier tube, as distinguished from the cascade or staged relation above referred to, the impedance or resistance R will have a magnitude preferably of the order of twice that given by the foregoing empirical formula and example.

The use of the resistance R in the input circuit of a detector tube, as V2, and in other relations where currents of radio or audio frequency are to be amplified, is of advantage notwithstanding there may not be present sharp resonance or tuning, as in the case of the anode circuit of the detector tube which is not sharply tuned but is at best broadly tuned. The utilization of the resistance R, however, finds its principal application in the input circuits of tubes, such as V and V1, used simply as radio frequency amplifiers.

It is preferred, as to each of the input circuits with which the resistance R is associated, that the grid g be negative, or at any rate not sumciently positive to effect substantial conductivity between grid and cathode f.

It is further preferable that for the wave lengths for which the receiving apparatus is to be utilized, or throughout a part or whole of a range of different wave lengths for which it is to be utilized, the inductance associated with each of the condensers C, C1 and C2 and constituting the secondary of the transformers T, T1 and T2 shall be the predominating frequency-determining element of the tuned circuit per se in the sense that such inductance is large compared with the associated tuning condenser, whereby the persistency of each tuned circuit is high or its decrement is small, with the result that such damping effect as the resistance R may have upon the tuned circuit is suitably small.

For example, for a wave length range of the order of from 200 to 600 meters, the secondary of each of the transformers T, T1 and T2 may have an inductance of the order of .3 millihenry, and each of the associated tuning capacities C, C1 and C2 may have a magnitude varying from about .00009 microfarad for 200 meters to about .00027 for 600 meters. From these relations it will be noted of each product L, C that the factor L greatly predominates.

The mutual inductance between the primary and secondary of each of the transformers T,

T1 and T2 may be of the order of .025 millihenry and the coupling co-eflicient of the order of per cent.

The proportions of the order and character above stated with respect to the coupling transformers involve step-up ratios of turns and voltage, and in the example given, the inductance of the primary is of the order of .02 or .03 millihenry, the step-up of voltage is of the order of 1 to 10, the secondary voltage being of the order of ten times that of the primary. Furthermore the coupling between primary and secondary windings is loose, in the sense that it is substantially and materially less than unity coupling or very close coupling.

A fixedinductance, as a winding of a coupling transformer, with its variable condenser constitutes a closed or loop circuit tunable throughout a wave length range and comprised in an input or output system; and across the terminals of the inductance or the tuning condenser are connected the cathode and one of the other electrodes of a tube. The resistance R is external to the tuned loop. This particular position of the resistance, external to the tuned loop or circuit, in series with the small capacity between grid and cathode, in a path in shunt to the loop, is of importance and advantage, as distinguished from connection of the resistance in series within the tunable closed or loop circuit, or of resistance in shunt to the loop without the small capacity.

By the external shunt connection of the resist ance in series with small capacity it becomes markedly effective in reducing the tendency to increased instability with increasing frequency,

resistance is used in a generic sense to include non-inductive resistance and resistance which may be more or less inductive.

What I claim is:

The combination with a thermionic amplifier, having grid, cathode and anode, of'an input system therefor tunable throughout a wave length range by fixed inductance and a variable condenser in shunt therewith forming a tunable loop, an output system tunable throughout said wave length range by a fixed inductance with a variable condenser eiTectively in shunt therewith, the tendency of the amplifying system to become unstable increasing throughout said wave length range as said input and output systems are tuned to shorter and shorter wave lengths of said range, means for biasing said grid to a potential substantially negative with respect to said cathode, whereby the conductivity between said grid and cathode is insubstantial, and a conductive impedance of fixed magnitude in said input system external to said tunable loop thereof and in series with capacity formed by said grid and cathode, whereby the tendency of the amplifying system to instability is compensated for by said impedance whose stabilizing efiect rapidly increases with decrease of wave length.

ALBERT D. SILVA. 

